1.2. Notes:

2: In American English, when writing out natural numbers of three or more digits, the word "and" is not used after "hundred" or "thousand". So it is "one hundred twenty-three" and not "one hundred and twenty-three", though you may hear a lot of people using the last, informally. In British English, the word "and" is used after "hundred" or "thousand" in numbers of three or more digits.

3. Do not use commas when writing out numbers above 999: so it is "one thousand two hundred thirty-four" and not "one thousand, two hundred thirty-four".

4. For clarity, use commas when writing figures of four or more digits: 1,234, 43,290,120, etc. In other countries a point is used to group digits by 3 and a comma to separate the decimals, ex: 1.234,55, 43.290.120,84. In some other countries a space is used to group digits by 3, ex: 1 234, 43 290 120.

2. When to write out numbers using words?

Spell out all numbers beginning a sentence, "Forty years ago today,..." Not "40 years ago today,...".

The Chicago Manual of Style calls for the numbers zero through one hundred to be written out - this would include forms like "one hundred million".

Using words to write short numbers makes your writing look clean and classy. In handwriting, words are easy to read and hard to mistake for each other. Writing longer numbers as words isn't as useful, but it's good practice while you're learning.

Otherwise, clarity should matter, for example when two numbers are used in a row: "They needed five 2-foot copper pipes to finish the job. There were 15 six-foot tall men on the basketball team roster.".

Be consistent within a sentence. Do not write "... one million people..." but "... 1,000,000 cars...", stick to one or another, but not both.